Cyclical DC voltage converter for suppressing voltage spikes and oscillations

ABSTRACT

A DC voltage converter is provided that cyclically converts an input-side supply voltage into an output voltage. The converter includes an inductive storage element connected between a terminal for the supply voltage and, in a manner such that it is coupled via a first switch, a reference potential terminal. The capacitively buffered output voltage terminal is connected, via a second switch, between the inductive storage element and the first switch. Provision is furthermore made of a third switch, which is intended to selectively short the inductive storage element that is connected in parallel and is controlled by a control circuit. The control circuit is controlled, on the input side, by a control voltage that is tapped off at the second terminal of the inductive storage element.

REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of the priority date of Germanapplication DE 10 2004 031 395.4, filed on Jun. 29, 2004, the contentsof which are herein incorporated by reference in their entirety.

FIELD OF THE INVENTION

The present invention relates to a DC voltage converter, which isoperated cyclically with an inductive storage element that can beshorted by means of a suitable control circuit in order to suppressvoltage spikes and voltage oscillations which occur during operation.

BACKGROUND OF THE INVENTION

DC voltage converters are used in portable devices in which thebatteries provide only a low supply voltage on account of the desiredminiaturization and weight saving. In order to supply the circuit unitsof the devices, a DC voltage converter converts the supply voltage intoa higher output voltage. The design of such a DC voltage converter(which is also referred to as a step-up converter) is described inTietze/Schenk: “Halbleiterschaltungstechnik” [Semiconductor circuittechnology], 12th edition, pages 948 to 949.

A DC voltage converter has an inductive storage element, which isconnected between a terminal for the supply voltage and, in such amanner that it is coupled via a first switch, a terminal for thereference potential. A capacitively buffered terminal for the outputvoltage is connected, via a second switch, between the inductive storageelement and the first switch.

During ideal cyclic operation, the first switch and the second switchchange over simultaneously, with the result that either the first switchis on and the second switch is off or the first switch is off and thesecond switch is on, respectively. If the first switch is on, energy isstored in the inductive storage element. The charge is removed if thesecond switch is on, and the capacitor is charged. If the switchingstates of the first and second switches remain unchanged, the coilcurrent falls continuously until the inductive storage element has beendischarged.

If the first and second switches are on, the energy stored in thecapacitor drains via the second and first switches. This impairs theefficiency of the DC voltage converter.

If the first and second switches are off at the same time and energy isstill stored in the inductive storage element, the coil current isinterrupted and voltage spikes which may damage the circuit occur. Inaddition, interfering oscillations occur, said oscillations being causedby the resultant resonant circuit that is formed from the inductivestorage element and the parasitic switch capacitance.

In practice, the first and second switches cannot be changed over atexactly the same time on account of propagation times and other effects.Therefore, it is difficult to avoid the first and second switches beingoff at the same time. The first and second switches may likewise be onat the same time. This state is usually avoided on account of theefficiency being greatly reduced. In addition, it is also sometimesdesired, when regulating the circuit, that the first and second switchesare off at the same time so that the output voltage assumes a prescribedvalue. In order to suppress the associated voltage spikes or voltageoscillations, the inductive storage element is shorted, with the resultthat the coil current flows in the short-circuit circuit. Such a circuitis also referred to as a snubber circuit. A snubber circuit has hithertousually been formed by a series circuit comprising a diode and a seriesresistor, said series circuit being connected in parallel with theinductive storage element. The series resistor is used to set a voltagevalue, at which the inductive storage element is shorted. The voltagevalue may not be less than the threshold voltage of the diode, which, inintegrated circuits, is possibly already a critical value.

The problem is that both the diode and the resistor must be dimensionedas accurately as possible (which is associated with outlay in terms oftechnology and costs) so that, on the one hand, the inductive storageelement is shorted at a desired threshold value and, on the other hand,the losses on account of the voltage drop across the resistor are nottoo large.

SUMMARY OF THE INVENTION

The following presents a simplified summary in order to provide a basicunderstanding of one or more aspects of the invention. This summary isnot an extensive overview of the invention, and is neither intended toidentify key or critical elements of the invention, nor to delineate thescope thereof. Rather, the primary purpose of the summary is to presentone or more concepts of the invention in a simplified form as a preludeto the more detailed description that is presented later.

The invention is directed to a DC voltage converter whose inductivestorage element is shorted by an individual switch having a suitablecontrol circuit if voltage spikes or voltage oscillations occur, whereinthe switch is implemented in a simple manner and generates few losses.

According to one embodiment of the invention, a DC voltage converter isdisclosed that cyclically converts an input-side supply voltage into anoutput voltage. The converter comprises an inductive storage elementhaving a first terminal and a second terminal, the first terminal ofwhich is connected to a supply voltage terminal.

The converter also includes a first switch which has a first terminaland a second terminal and is connected in series with the inductivestorage element, wherein the first terminal of the first switch isconnected to the second terminal of the inductive storage element andthe second terminal of the first switch is connected to a referencepotential terminal.

The converter further comprises a second switch having a first terminaland a second terminal, wherein the first terminal thereof is connectedto the second terminal of the inductive storage element and the secondterminal is connected to an output voltage terminal.

In addition, a capacitive storage element is provided, which isconnected between the output voltage terminal and the referencepotential terminal. Lastly, a third switch is provided, which isconfigured to short the inductive storage element, is connected inparallel with the inductive storage element and is connected to acontrol circuit, which can be controlled, on the input side, by acontrol voltage that is tapped off at the second terminal of theinductive storage element.

In the DC voltage converter according to one embodiment of theinvention, the inductive storage element is shorted by the activatablethird switch if the control circuit detects voltage spikes or voltageoscillations. This feature has the advantage that the third switch hasonly low power losses and both the third switch and the components ofthe control circuit can be formed in an inexpensive and simple mannerusing integrated circuit technology.

One additional embodiment of the control circuit comprises a thresholdvalue decision unit, to which the control voltage is applied on theinput side, and a downstream storage element whose output-side storagesignal is coupled to a control terminal of the third switch. Thisembodiment has the advantage that it comprises only two essentialstandard components which can be formed in an inexpensive and simplemanner using integrated circuit technology.

Another embodiment of the invention comprises the provision of aregulating circuit, to which the output voltage is applied on the inputside and which provides a first switching signal and a second switchingsignal on the output side so that the output voltage assumes aprescribed value. This means that the prescribed output voltage isprovided even when the load is changed.

In one example the first and second switching signals are provided sothat the first switch and the second switch are on in the push-pull modeor are off in the push-pull mode. This corresponds to idealhigh-efficiency operation of the DC voltage converter.

In another example the first switch is in the form of an n-channel fieldeffect transistor and the second switch is in the form of a p-channelfield effect transistor or the first switch is in the form of ap-channel field effect transistor and the second switch is in the formof an n-channel field effect transistor, and the first switching signaland the second switching signal are in phase or are virtually in phase,with the result that both switches can also be driven using the samesignal.

In a DC voltage converter whose first switch and whose second switch arein the form of n-channel field effect transistors or whose first switchand whose second switch are in the form of p-channel field effecttransistors, the first switching signal and the second switching signalare in antiphase or are virtually in antiphase. This has the advantagethat only one type of switch is used.

In an alternative embodiment, the second switch of the DC voltageconverter is in the form of a diode, as shown in FIG. 3. In this case,the regulating circuit provides only the first control signal.

In the control circuit, a threshold value decision unit assigns one oftwo logic states to an output-side signal. A first logic state isassumed if an internal threshold value that is greater than a prescribedoutput voltage is exceeded, and a second logic state is otherwiseassumed. This is expedient in order to detect voltage spikes whichoccur.

In an alternative embodiment of the threshold value decision unit, thethreshold value may be selected in such a manner that undershootsoriginating from the resonant circuit which may occur are detected.

In one embodiment, the threshold value decision unit has two internalthreshold values, a first threshold value for changing over from thefirst to the second state, by switching hysteresis, and a secondthreshold value for changing over from the second to the first state.This aspect may also be referred to as a Schmitt trigger and can beimplemented in a particularly simple manner.

The storage element used in the control circuit in one example has a setinput, which is coupled to the output of the threshold value decisionunit, a reset input and an output, the reset input being coupled to thefirst switching signal in such a manner that one of the logic states isapplied to the storage element on the output side if the first switch ison and another logic state is applied on the output side as soon as aclock edge appears at the set input if the first switch is off. Theoccurrence of a first overshoot or undershoot is indicated by changingthe initial state. This initial state remains unchanged if the firstswitch is off. The output signal is advantageously coupled to the thirdswitch that shorts, with the result that the inductive storage elementis shorted when voltage oscillations occur.

In one embodiment, the output of the storage element is coupled to thethird switch via an inverter. The inverter comprises a driver inverterthat switches quickly as regards the gate capacitance (the charge ofwhich is to be reversed) of the third switch.

In accordance with another embodiment, the storage element comprises aD-type flip-flop whose integration is sufficiently well known.

In one embodiment of the control circuit, provision is made of a firstinverter, which is connected upstream of the reset input of the storageelement, with the result that the storage element is reset if the firstswitch is on.

The threshold value decision unit may be provided with a supply input.The storage element may be formed such that it has a further input,which, for example in the case of the D-type flip-flop, is used toassign a value to one of the logic states.

The third switch may comprise an n-channel field effect transistor orp-channel field effect transistor. This results in degrees of freedomwhen designing the circuit.

In another example the DC voltage converter is designed using integratedcircuit technology. The further miniaturization of devices is thussupported.

To the accomplishment of the foregoing and related ends, the inventioncomprises the features hereinafter fully described and particularlypointed out in the claims. The following description and the annexeddrawings set forth in detail certain illustrative aspects andimplementations of the invention. These are indicative, however, of buta few of the various ways in which the principles of the invention maybe employed. Other objects, advantages and novel features of theinvention will become apparent from the following detailed descriptionof the invention when considered in conjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is explained below with reference to the drawing, inwhich:

FIG. 1 is a block level diagram illustrating one exemplary embodiment ofthe invention;

FIG. 2 is a combined timing and state diagram illustrating the timeprofile of selected signals; and

FIG. 3 is a block level diagram illustrating an alternative exemplaryembodiment of the Invention.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 shows one exemplary embodiment of the invention, which isoperable to convert a supply voltage Vin (which is applied initially)into an output DC voltage Vout. An inductive storage element 1 having afirst terminal 11 and a second terminal 12 is coupled, by way of thefirst terminal 11, to a terminal for providing the supply voltage Vinand is connected, by way of the second terminal 12, to a referencepotential Vss via a switch 2. A second switch 3 having a first terminal31 and a second terminal 32 is connected, via the first terminal 31, tothe second terminal 12 of the inductive storage element 1. The secondterminal 32 of the second switch 3 is connected to a terminal fortapping off the output voltage Vout. A capacitor 4 is connected betweenthe terminal for tapping off the output voltage Vout and the referencepotential Vss. A third switch 5 for shorting the inductive storageelement 1 is connected in parallel with the latter. The third switch 5can be switched using a control circuit 6 whose input-side controlvoltage V1 is tapped off at the second terminal 12 of the inductivestorage element 1.

The control circuit 6 comprises a threshold value decision unit 7, whichhas a storage element 8 connected downstream of it, the output of saidstorage element being coupled to the control input of the third switch5. As shown in FIG. 1, a second inverter 11 is provided for couplingpurposes. The inverter may also be in the form of a fast-switchingdriver inverter. As an alternative, it is also possible, for example, tocouple directly or via an amplifier. The threshold value decision unit 7is formed in such a manner that a first state is applied to the outputof the threshold value decision unit 7 if the input signal exceeds aninternal threshold and in such a manner that a second state of thethreshold value decision unit output is otherwise applied.

An alternative embodiment of the threshold value decision unit 7, whichmay be, for example, a Schmitt trigger, has two internal thresholdvalues, a first threshold value for changing over from the first to thesecond state being distinguished, by switching hysteresis, from a secondthreshold value for changing over from the second to the first state.

The storage element 8 has a set input, a reset input, and an output, andis driven, using the first switching signal S2, in such a manner thatone logic state is applied to the output side if the first switch 2 ison and in such a manner that another logic state is applied to theoutput side if a clock edge appears at the set input and the firstswitch 2 is off. In FIG. 1, a first inverter 10 is provided for thepurpose of coupling the reset input to the first switching signal S2.

One advantageous exemplary implementation of the storage element 8 is aD-type flip-flop whose initial state is reset if no voltage is appliedto the reset input, as illustrated by way of example in FIG. 1. Thisembodiment allows the circuit to be started up in a stable manner.

The threshold value decision unit may be provided with a supply input.The storage element may be formed such that it has a further input,which, for example in the case of the D-type flip-flop, is used toassign a value to one of the logic states. This input, in one example,is connected to the output voltage Vout. As shown in FIG. 1, thethreshold value decision unit 7 is formed, by way of example, without asupply input and the storage element 8 is formed such that it has afurther input.

In addition, provision is made of a regulating circuit 9 whose functionis to ensure that the output voltage Vout assumes a prescribed value.The output voltage Vout is applied to the regulating circuit 9 on theinput side and a first switching signal S2 for controlling the firstswitch 2 and a second switching signal S3 for controlling the secondswitch 3 are provided on the output side.

The first switching signal S2 and the second switching signal S3 areideally selected in such a manner that the first switch 2 is on and thesecond switch 3 is off or the first switch 2 is off and the secondswitch 3 is on. Suitable embodiments of the switches are n-channel fieldeffect transistors or p-channel field effect transistors. In FIG. 1, thefirst switch 2 is, for example, in the form of an n-channel field effecttransistor and the second switch 3 is, for example, in the form of ap-channel field effect transistor. In this case, the first switchingsignal S2 and the second switching signal S3 are in phase or arevirtually in phase. This means, for example, that the first switchingsignal S2 and the second switching signal S3 simultaneously have a highsignal level and simultaneously have a low signal level. It is likewiseconceivable for the first switch 2 to be in the form of a p-channelfield effect transistor or for the second switch 3 to be in the form ofan n-channel field effect transistor.

In order to describe the method of operation of the control circuit 6,the first switching signal S2 and the second switching signal S3 are,for example, selected in such a manner that three phases occur duringcircuit operation:

-   -   in phase I, the first switch 2 is on and the second switch 3 is        off,    -   in phase II, the first switch 2 is off and the second switch 3        is on, and    -   in phase III, both the first switch 2 and the second switch 3        are off.

In addition, another phase IV is possible, in which both the firstswitch 2 and the second switch 3 are on. Phase I, in which the inductivestorage element 1 is charged, and phase II, in which the capacitor 4 ischarged, are essential to operation of the DC voltage converter. Voltagespikes and voltage oscillations which are attenuated by using thecontrol circuit 6 may occur during phase III. During phase IV, thecapacitor is discharged via the second switch 3 and the first switch 2,thus impairing the efficiency but not resulting in voltage spikes orvoltage oscillations which could be attenuated by the control circuit 6.This situation is therefore not considered in any more detail.

FIG. 2 uses a state diagram to illustrate the temporal change inselected time signals. The first switching signal S2, the secondswitching signal S3, the control voltage V1, the output signal S7 of thethreshold value element 7 and the third switching signal S5 that islinked to the output of the storage element 8 are illustrated.

The first switching signal S2 cyclically changes its state. It iscoupled to the first switch 2 in such a manner that, in the case of alow level, the first switch 2 is off and, in the case of a high level,the first switch 2 is on. The same assignment of the levels determinesthe switching state of the third switch 5. The second switching signalS3 is coupled to the second switch 3 in such a manner that, in the caseof a low level, the second switch 3 is on and, in the case of a highlevel, the second switch 3 is off.

In phase I, the inductive storage element 1 is charged by a current viathe terminal for the supply voltage Vin. The control voltage V1 that isapplied to the input of the control circuit 6 is at reference potentialVss. The output signal S7 of the threshold value decision unit 7 is setto a first logic state of the threshold value decision unit output. Thereset input of the storage element 8 is coupled to the first switchingsignal S2 in such a manner that a first logic state of the storageelement output is applied. It should be noted that the first logic stateof the threshold value decision unit output does not have to have thesame value as the first logic state of the storage element output.

The output of the storage element 8 is coupled to the third switch 5 insuch a manner that the latter is off during the first logic state of thestorage element output.

In phase II, a discharging current of the inductive storage element 1flows via the second switch 3 and the capacitor 4 is charged. Thecontrol voltage V1 is at approximately the same potential as the outputvoltage Vout.

The internal threshold value of the threshold value decision unit 7 isset in such a manner that either overshoots or undershoots of thecontrol voltage V1 are detected. If the threshold value is set to detectovershoots, the first state of the threshold value decision unit outputis applied. If the threshold value is set to detect undershoots, asecond logic state of the threshold value decision unit output isapplied. In this exemplary embodiment, the threshold value decision unitis designed in such a manner that its output is inverting and itsinternal threshold is set to detect undershoots. In this case, in phaseII, the output signal S7 is in the second state of the threshold valuedecision unit output.

The output of the storage element 8, like the third switching signal S5,remains unchanged since the first switch 2 is off and no clock edgeappears on the input side. The third switch 5 remains off.

If, in phase III, energy is still stored in the inductive storageelement 1, the flow of current is interrupted if the first switch 2 andthe second switch 3 are simultaneously off. The control voltage V1 wouldhave voltage spikes and voltage oscillations if it were not attenuatedby shorting (in accordance with the invention) the coil current when afirst overshoot or undershoot occurred.

When an overshoot of the control voltage V1 occurs, the second state ofthe threshold value decision unit 7 is applied to the output of thelatter if the internal threshold value has been set to detectovershoots. When an undershoot of the control voltage V1 occurs, thefirst state of the threshold value decision unit 7 is applied to theoutput of the latter if the internal threshold value has been set todetect undershoots.

In FIG. 2, the control voltage V1 first of all rises and then falls inphase III. When the internal threshold for detecting undershoots isundershot, the output signal S7 of the threshold value decision unit 7changes to the first state.

When a detected overshoot or a detected undershoot occurs, theoutput-side state of the storage element 8 changes since the firstswitch 2 is off and a signal edge occurs on the input side. Theoutput-side state of the storage element 8 remains in this state sincefurther clock edges do not change the state if the first switch 2 isoff. Consequently, the third switching signal S5 has a high level andthe third switch 5 is on. As a result, the inductive storage element 1is shorted, with the result that further oscillations of the controlvoltage V1 are suppressed and the control voltage V1 is forced to thepotential of the supply voltage Vin.

While the invention has been illustrated and described with respect toone or more implementations, alterations and/or modifications may bemade to the illustrated examples without departing from the spirit andscope of the appended claims. In particular regard to the variousfunctions performed by the above described components or structures(assemblies, devices, circuits, systems, etc.), the terms (including areference to a “means”) used to describe such components are intended tocorrespond, unless otherwise indicated, to any component or structurewhich performs the specified function of the described component (e.g.,that is functionally equivalent), even though not structurallyequivalent to the disclosed structure which performs the function in theherein illustrated exemplary implementations of the invention. Inaddition, while a particular feature of the invention may have beendisclosed with respect to only one of several implementations, suchfeature may be combined with one or more other features of the otherimplementations as may be desired and advantageous for any given orparticular application. Furthermore, to the extent that the terms“including”, “includes”, “having”, “has”, “with”, or variants thereofare used in either the detailed description and the claims, such termsare intended to be inclusive in a manner similar to the term“comprising”.

1. A DC voltage converter, which cyclically converts an input-sidesupply voltage into an output voltage, comprising: an inductive storageelement comprising a first terminal and a second terminal, wherein thefirst terminal is coupled to a supply voltage terminal; a first switchcomprising a first terminal and a second terminal, and coupled in serieswith the inductive storage element, wherein the first terminal of thefirst switch is coupled to the second terminal of the inductive storageelement and the second terminal of the first switch is coupled to areference potential terminal; a second switch comprising a firstterminal and a second terminal, wherein the first terminal thereof iscoupled to the second terminal of the inductive storage element and thesecond terminal thereof is coupled to an output terminal at which anoutput voltage is provided; a capacitive storage element coupled betweenthe output terminal and the reference potential terminal; and a thirdswitch configured to selectively short the inductive storage element,and coupled in parallel with the inductive storage element, andconfigured to be controlled by a control circuit that is configured togenerate a control signal based on a control voltage tapped off at thesecond terminal of the inductive storage element, the control circuitcomprising a threshold value decision unit to which the control voltageis applied on an input side thereof, and a downstream storage elementconfigured to output the control signal to a control terminal of thethird switch.
 2. The DC voltage converter of claim 1, wherein the firstswitch is configured to switch cyclically in response to a firstswitching signal, and the second switch is configured to switchcyclically in response to a second switching signal.
 3. The DC voltageconverter of claim 1, further comprising a regulating circuit configuredto receive the output voltage and generate the first switching signaland the second switching signal in response thereto so that the outputvoltage assumes a prescribed value.
 4. The DC voltage converter of claim1, wherein the regulating circuit generates the first switching signaland the second switching signal such that the first switch and thesecond switch are on in a push-pull mode or are off in the push-pullmode.
 5. The DC voltage converter of claim 4, wherein the first switchcomprises an n-channel field effect transistor and the second switchcomprises a p-channel field effect transistor, or the first switchcomprises a p-channel field effect transistor and the second switchcomprises an n-channel field effect transistor, and wherein the firstswitching signal and the second switching signal are in phase or aresubstantially in phase.
 6. The DC voltage converter of claim 4, whereinboth the first switch and the second switch comprise n-channel fieldeffect transistors or p-channel field effect transistors, respectively,and wherein the first switching signal and the second switching signalare in antiphase or are substantially in antiphase.
 7. The DC voltageconverter of claim 1, wherein the threshold value decision unit isconfigured to assign one of two logic states to an output-side signalprovided to the downstream storage element, wherein a first logic stateis provided if an internal threshold value for detecting overshoots isexceeded and a second logic state is provided otherwise.
 8. The DCvoltage converter of claim 1, wherein the threshold value decision unitis configured to assign one of the two logic states to an output-sidesignal, wherein a first logic state is provided if an internal thresholdvalue for detecting undershoots is undershot and a second logic state isprovided otherwise.
 9. The DC voltage converter of claim 7, wherein thethreshold value decision unit comprises two internal threshold valuesassociated therewith, wherein a first threshold value for changing overfrom the first to the second state is distinguished, by switchinghysteresis, from a second threshold value for changing over from thesecond to the first state.
 10. The DC voltage converter of claim 1,further comprising a downstream storage element, wherein the downstreamstorage element comprises a set input coupled to an output of thethreshold value decision unit, a reset input and an output, wherein thereset input is coupled to the first switching signal such that one ofthe logic states is applied to the storage element on the output sidethereof if the first switch is on and another logic state is applied onthe output side as soon as a clock edge appears at the set input if thefirst switch is off.
 11. The DC voltage converter of claim 10, furthercomprising a first inverter coupled upstream of the reset input of thedownstream storage element, and configured to reset the storage elementif the first switch is on.
 12. The DC voltage converter of claim 11,wherein the output of the storage element is coupled to the third switchand is configured to turn on the third switch in response to a clockedge appearing at the set input of the storage element and the firstswitch being off.
 13. The DC voltage converter of claim 10, furthercomprising a second inverter coupled downstream of the storage elementand configured to drive the third switch, wherein the third switch is onif a clock edge appears at the set input of the storage element and thefirst switch is off.
 14. The DC voltage converter of claim 1, whereinthe downstream storage element comprises a D-type flip-flop and has afurther input configured to assign a value to one of the logic states,and wherein the input is coupled to the output voltage terminal.
 15. TheDC voltage converter of claim 1, wherein the third switch comprises ann-channel field effect transistor or a p-channel field effecttransistor.
 16. The DC voltage converter of claim 1, wherein the secondswitch comprises a diode having an anode coupled to the second terminalof the inductive storage element and a cathode coupled to the outputvoltage terminal.
 17. The DC voltage converter of claim 1, wherein thethreshold value decision unit and the storage element each have a supplyterminal coupled to the output voltage terminal.
 18. The DC voltageconverter of claim 1, wherein the second terminal of the second switchis directly coupled to the output terminal at which an output voltage isprovided.
 19. The DC voltage converter of claim 1, wherein the thirdswitch is configured to provide a shorting path from the supply voltageterminal to the first terminal of the first switch and the firstterminal of the second switch, wherein the shorting path bypasses allinductive elements between the supply voltage terminal and the first andsecond switches.